Customization of the symmetrization surface matching procedure (Greve et al., 2013) based on the removal of sulcal position asymmetries and the use of a symmetrical template.
FreeSurfer processes each hemisphere independently of the other and therefore constructs models of the cortical surface that differ between the two hemispheres, thus preventing direct left-right matching of these surface models.
To establish surface-based asymmetry maps, a precise alignment of vertices characterizing the left and right surfaces is therefore needed. To do this, we customized the symmetrization surface matching procedure proposed by Greve et al. (2013) that is based on the removal of sulcal position asymmetries and the use of a symmetrical template (LH_Sym).
Inter-hemispheric surface-based registration for vertex-wise asymmetrical analysis. Left part of panel A illustrated left (respectively right) mean surface rendering and mean spherical projection of 40 subjects used for computing the symmetrical template. Red (respectively yellow) colored areas in the spherical projection correspond to sulci in the left (respectively right) hemispheres. Right part of panel A illustrates the sulci misalignment between hemispheres before symmetrization. Panel B shows the same features after symmetrization illustrating that left and right homotopic sulci are quite perfectly aligned and that the cortical asymmetry analysis is not bias due to sulci shape and position differences between hemispheres. LH: left hemisphere, RH: right hemisphere, STS: superior temporal sulcus. Derived from Maingault et al 2016.
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Greve DN, Van der Haegen L, Cai Q, Stufflebeam S, Sabuncu MR, Fischl B, Brysbaert M. A surface-based analysis of language lateralization and cortical asymmetry. J Cogn Neurosci. 2013 Sep;25(9):1477-92. doi: 10.1162/jocn_a_00405